CO2 lasers are commonly used in commercial manufacturing for operations such as cutting or drilling, in particular, in nonmetallic materials. One form of CO2 laser suited for such operations is known to practitioners of the art as a “slab” laser. Such a laser has an assembly including a pair of elongated, slab-like planar electrodes arranged face-to-face and spaced apart to define a gap between the electrodes. The electrodes are usually contained in a gas tight enclosure. The enclosure and the gap between the electrodes are filled with a lasing gas mixture including CO2. A radio frequency (RF) potential is applied across the electrodes to cause an electrical discharge in the CO2 laser gas mixture. The discharge energizes the CO2 lasing gas. A pair of mirrors is arranged, with one thereof at each end of the pair of electrodes, to form a laser resonator. A preferred type of resonator is a hybrid waveguide-unstable resonator. The energized CO2 lasing gas provides optical gain allowing laser radiation to be generated in the resonator. The electrodes form a waveguide or light guide for the laser radiation in an axis of the resonator perpendicular to the plane of the electrodes. This confines the lasing mode of the resonator in that axis. The mirrors define the lasing mode in an axis parallel to the plane of the electrodes. In an unstable resonator arrangement, laser radiation is delivered from (in effect, spilled out of) the resonator by bypassing one of the resonator mirrors.
In a slab laser used for drilling, cutting, and other machining operations a high output power, for example, greater than about 100 Watts (W), and maximum possible efficiency are important. In any given slab laser configuration, available average output power generally increases with increasing gas pressure and RF input power, provided that the average RF input power does not heat the gas discharge beyond 600° K. Further, when operating in a pulsed-mode, significant increases in peak RF input power are possible with increasing gas pressure provided that a corresponding increase in peak optical output power is available. This peak power increase is available only provided that the pulse delivery duty cycle is delivered to limit the RF input power as previously discussed. With the increase in gas pressure, faster optical rise and fall times (of pulses) occur due to corresponding increases in molecular collision rates of excited species in the discharge and consequential increases in the decay rate of the upper excited laser level.
Generally, an upper limit in peak optical output power available for a specific slab laser configuration, operating in a pulsed mode, at a specific duty cycle and pulse repetition frequency is defined by stability of the RF discharge itself as peak RF input power is increased. As peak RF power is increased, a critical point is reached at which the RF discharge can collapse from a low current form to a power limiting form and can collapse further into damaging arcs. This adversely affects the mode quality and efficiency of the laser. There is a need for an improvement of discharge stability in slab lasers operating in a pulsed mode with very high peak input powers to achieve high peak optical powers from compact slab laser sources.